I'd advise you to avoid buying any wheel without first trying one to see if it will actually fit, even if the measurements say they might. I doubt 16" wheels will fit over your rear brakes.
Wheel offset is given as the distance the mounting surface is shifted from the centerline of the wheel, lots for dual wheels and little to none for most single wheels.
Second is the lock ring wheels aren't the end of the world, don't be unnecessarily afraid of them.
There is a little info on the different types of wheels here. The RH5 wheels are the ones to stay away from and I'm nearly certain your's aren't.
http://www.stovebolt.com/techtips/wheels/One thing that's nice about the AD 1 ton dual wheel trucks are the distinctive 8 hand hole 18" wheels, if you run modern wheels you'll lose that.